In the production of mineral insulation elements, a low thermal conductivity (TC) and a low bulk density combined with a relatively high mechanical strength are sought.
DE 101 31 360 A1 describes a process for producing porous concrete insulation slabs comprising aluminum in the form of paste as pore former, with, for example, bulk densities of from 100 to 120 kg/m3 being achieved at a thermal conductivity of λ=0.045 W/mK and a compressive strength of 0.5 N/mm2.
In the case of blowing by means of aluminum, e.g. in the form of aluminum powder or paste, pores which have been blown to a different extent occur in the cast fresh compositions with decreasing distance from the surface as a result of the decreasing hydrostatic pressure. The porosity distribution in the insulation elements obtained is thus not homogeneous. The relatively large pores prevent thermal conductivities of less than, for example, 0.045 W/mK from being achieved.
In addition, relatively large blown pores are obtained, so that, for this reason too, the desired relatively low thermal conductivities cannot be achieved.
It is an object of the present invention to provide a mineral aluminum-blown insulation element which is bound by means of calcium silicate hydrate and has a low thermal conductivity of, in particular, less than 0.045 W/mK and a high strength combined with a low bulk density, in particular from an inexpensive mix, and also a process for producing it.
This object is achieved by the compositions and methods of this invention.
To achieve the objects of this invention, an attempt was firstly made to bring about formation of small pores by use of finer aluminum particles than usual, e.g. in a paste or in the form of a powder. However, it was found that the finer aluminum particles are strongly agglomerated before the reaction with water which forms hydrogen bubbles, leading to even larger pores than when aluminum particles having a customarily used particle size are used.
The invention has therefore taken a novel, nonobvious route by using generally platelet-like aluminum particles of customary fineness and inhibiting bubble formation and/or ensuring that the aluminum particles are largely deagglomerated and attached to solid particles of the suspension of the fresh composition. This results in formation of smaller blown pores than in processes of the prior art, so that smaller thermal conductivities can also be achieved and, in addition, smaller bulk densities can be made available at adequate to very good strengths.
The invention accordingly provides for the use of agents which develop small blown pores by, for example, an affinity of aluminum particles or small aluminum particle agglomerates present in the aluminum products to the solid particle surface in the suspension of the fresh composition being produced and a certain degree of bonding being effected. Instead of this or in combination therewith, agents which have a deagglomerating action in respect of the aluminum particles can also be used. As a result, the formation of blown pores is surprisingly inhibited or hindered in such a way that the different hydrostatic pressure in a suspension is largely without influence. For the purposes of the invention, the agents will also be referred to below as “blown pore inhibitors”.